"I have decreed many decrees upon myself" — Eruvin 21b
R' Yosef Engel (Gilyonei HaShas) finds this assertion of the Gemara difficult. It would seem that Rabbinic decrees are not issued by Knesses Yisrael — the Jewish people upon themselves, but rather by their leaders, the Sages, upon them?
Initially R' Yosef Engel suggests that perhaps Knesses Yisrael refers to the Sages themselves, not to the general population. As evidence he cites Rashbam to Pesachim 118b (d.h. Amrah Knesses Yisrael) who indicates this to be the case.
Alternatively, he suggests, the Sages are the representative legislative body of the general population, and hence any decree they enact is as it if is enacted by the totality of Knesses Yisroel.
However, ultimately R' Yosef Engel concludes that the Gemara's assertion here is best understood in light of the principle that the Sages cannot enforce a decree that they decree unless the majority of the general population tolerates the decree [אין בית דין גוזרין גזירה על הציבור אלא א"כ רוב הציבור יכולין לעמוד בה] (see Yerushalmi, Avodah Zarah 2:8). This is the reason why the attempt made by the Sages to ban oil pressed by a non-Jew failed (see Igros Moshe, Orach Chaim 2:100 that this is also a reason why it may no longer be forbidden to clap and dance on Shabbos). Thus, the effectiveness of any decree is primarily determined by the popular sentiment and by popular acceptance. Hence, it is indeed Knesses Yisrael that decrees decrees upon itself